This document describes, how to include new 
distance functions into the general tree algebra, at the example point.


STEP 1 : Implement a mapping from Secondo data type to void* 
============================================================

1.1. Declare the function
---------------------------
In the file DistDataReg.h, declare a function like:

static DistData *getDataPoint(const void *attr);


1.2 implement the function
---------------------------

This step changes the file  DistDataReg.cpp 

Firstly, add missing include into this file

#include "Coord.h"
#include "Point.h"


Secondly, implement this function

DistData* DistDataReg::getDataPoint(const void* attr) {
   // cast to the correct type
   const Point* point = static_cast<const Point*>(attr);
   // special treatment for undefined values
   if(!point->IsDefined()){ // undefined points value
      return new DistData(0,0);
   }
   // serialize the point
   Coord x = point->GetX();
   Coord y = point->GetY();

   char buffer[2*sizeof(Coord)];
   memcpy(buffer, &x, sizeof(Coord));
   memcpy(buffer + sizeof(Coord), &y, sizeof(Coord));
   return new DistData(2*sizeof(Coord) , buffer);
}


1.3. Register this function
-----------------------------

Navigate to the function DistDataReg::initialize()
in file DistDataReg.cpp and add a call like:

addInfo(DistDataInfo(
     DDATA_NATIVE, DDATA_NATIVE_DESCR, DDATA_NATIVE_ID,
     Point::BasicType(), getDataPoint));

The first parameters are predefined values. If you are not hapy with them,
define new value and use them here.

The fourth parameter contains the Type name used in Secondo.
The last parameter is the function defined before.




STEP 2: Computing the distance
===============================

1.1 Declaring the function
--------------------------

Firstly, change the file DistfunReg.h and add a declaration of the distance function
and possible new descriptions.

static void euclidPoint(
            const DistData *data1, const DistData *data2,
            double &result);


1.2 implement the function in file DistfunReg.cpp
--------------------------

add missing includes
#include "Coord.h"
#include "Point.h"

void DistfunReg::euclidPoint(
         const DistData *data1, const DistData *data2,
         double &result) {

     // handle undefined values
     if(data1->size()==0 && data2->size()==0){
        result = 0;
        return ;
     }   
     if(data1->size()==0 || data2->size()==0){
        result =  numeric_limits<double>::max();
        return;
     }   

     Coord x1; 
     Coord y1; 
     memcpy(&x1, data1->value(), sizeof(Coord));
     memcpy(&y1, (char*) data1->value() + sizeof(Coord), sizeof(Coord));
     Coord x2; 
     Coord y2; 
     memcpy(&x2, data2->value(), sizeof(Coord));
     memcpy(&y2, (char*) data2->value() + sizeof(Coord), sizeof(Coord));
     result = sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2)); 
 }





1.3 register the function
-------------------------

navigate to function DistfunReg::initialize() in file DistfunReg.cpp.
and add a function call


 addInfo(DistfunInfo(
      DFUN_EUCLID, DFUN_EUCLID_DESCR,
      euclidPoint,
      DistDataReg::getInfo(Point::BasicType(), DDATA_NATIVE),
      DFUN_IS_METRIC | DFUN_IS_DEFAULT));

The first two parameters are the name and a description of the function.
If predefined names and descriptions are not sufficient, just add new definitions.

The third parameter is the distance function itself.


The next argument is build up from the name of the data type in secondo and the name 
in of the conversion function defined in step1.

The last parameter determines that this distance function describes a metric and is to use 
as the default function. See code for additional flags.